Subconjunctival fibrosis is a major complication associated with eye surgery. While several surgical procedures and medical implants are available to treat a number of diseases and disorders of the eye, resultant tissue fibrosis often leads to unsatisfactory post-operative outcomes.
Glaucoma is a multifactorial optic neuropathy in which there is a characteristic acquired loss of retinal ganglion cells and atrophy of the optic nerve. Major risk factors for glaucoma include elevated intraocular pressure, positive family history, African heritage, and older age. Reduction and control of the intraocular pressure remains the main stay of treatment in the management of glaucoma. The increase in the intraocular pressure is thought mainly due to outflow resistance. Elevated intraocular pressure can be reduced pharmacologically, following surgical filtration procedures (GFS) or by the use of glaucoma drainage devices (GDD). The success rate of these operations is about 70-80% at one year and 40-50% at five years.
Excessive postoperative fibrosis at the wound site significantly reduces surgical success following glaucoma surgery. Pharmacological attempts to prevent fibrosis following glaucoma surgery have thus far proven unsatisfactory. Topical steroids have been the mainstay for suppressing the inflammatory/fibrous reaction that follows the various glaucoma filtering surgeries, but with variable results. The use of antifibrotic medications or agents such as, for example, 5-fluorouracil (5-FU) and mitomycin-C (MMC), help prevent the postsurgical fibrosis and improve the surgical outcomes following trabeculectomy operation (85-90% at one year compared to 70% without these agents). However, the current technique of delivering the MMC or 5-FU in the form of soaking a small wedge of sponge in a given concentration of the drug and applying to the operation site for variable time periods before washing the drug from the surgical site leads to inconsistent results.
The inflammatory/fibrous reaction that follows an operation implanting a glaucoma drainage device differs from that of trabeculectomy in that the inflammatory reaction is ongoing probably due to the biomaterial of the glaucoma drainage device. This leads to bleb encapsulation and elevated intraocular pressures in both the short term, that is, in the hypertensive phase and in the long term leading to elevated intraocular pressure and failure of the operation. The failure rate for the glaucoma drainage devices has been reported at 10% per post-operative year, thus reaching 50% failure rate in 5 years.
Antifibrotic agents have also been used during glaucoma drainage device implant operations, but with variable and questionable results. Topical application of such medications at the time of surgery appears to be less effective 7-9 mm from the limbus for such implant operations compared to the effects seen at the limbus during a trabeculectomy operation. It is therefore possible that one-time application of these agents may not be sufficient to decrease the chances of long-term fibrous reaction that occurs following an implant operation.
In the recent past, non-penetrating filtering procedures like the viscocanulostomy and deep sclerectomy with or without a collagen implant are gaining popularity as a means of controlling the intraocular pressure, at the same time avoiding the immediate postoperative complications of a penetrating filtering procedure like the trabeculectomy such as hypotony, flat anterior chamber, choroidal effusions etc. The success rate of these operations, however, is less than 50% at 12 months because of scar tissue formation.
Canaloplasty is a form glaucoma surgery in which the Schlemm's canal is dilated 360° using a microcatheter (iSciences Interventional, Calif.) followed by the placement of a stent (10-0 Prolene) to exert tension on the dilated canal. The long-term results of this operation are still unknown. Schlemm's canal is a circular channel in the eye lined with a single layer of vascular-derived endothelial cells that transports 2-3 microliters of aqueous humor per minute from the anterior chamber to the venous blood supply. By microscopy, the canal measures between 190-350 microns in average diameter in human eyes. The canal serves as the final barrier before aqueous humor enters systemic circulation. This tissue has been extensively studied because of its importance in various ocular pathologies, including glaucoma. Canaloplasty procedure is one of the first procedures to enhance the aqueous drainage through the canal by mechanical stretching. However, the long-term success of this procedure (currently unknown) may be restricted because of reactionary fibrosis. This may be prevented by the use of a slow release drug coated stent (similar to the cardiac stents).
The inflammatory/fibrous reaction that follows implant operations defers from that of the trabeculectomy operation in that the inflammatory reaction is on going, probably due to the biomaterial that makes up the implanted glaucoma drainage device and the micro-motion exhibited by an end-plate with ocular movements and the presence of the aqueous medium in the subconjunctival space and a lot of other factors that are not well understood at the present time.
In glaucoma filtration non-penetrating surgery (NPS), excessive postoperative scarring at the wound site significantly reduces surgical success. Pharmacological attempts to prevent fibrosis following glaucoma surgery have thus far proven unsatisfactory. Topical steroids have been the mainstay for suppressing the inflammatory/fibrous reaction that follows the various glaucoma filtering surgeries, but with variable results. The use of antifibrotic agents such as, for example, 5-FU and MMC help prevent the postsurgical fibrosis and improve the surgical outcomes. However, the current technique of delivering 5-FU and MMC in the form of soaking a small wedge of sponge in a given concentration of the medication and applying to the operation site for a variable time period before washing the medication from the surgical site leads to inconsistent results. Therefore, there is a need for more specific treatments directed against the inflammatory/fibrous reaction both in terms of the ability to deliver the medication in a dose dependent and predictable fashion. Desirably such treatment may also be able to deliver new antifibrotic agents having less associated problems than the current ones in use.